The invention relates to a coupling device for connecting a towing vehicle to a semi-trailer, in which a kingpin is arranged on the trailer and a tractor coupling on the towing vehicle, said tractor coupling having locking elements for positive and rotatable grasping of substantially rotationally symmetrical surfaces of the kingpin, the surfaces being formed by an abrasion-resistant thermal coating.
A coupling device of the aforesaid kind is known from DE A1-43 04 857.
The invention further concerns a method for modifying coupling devices for connecting a towing vehicle to a semi-trailer, in which a kingpin is arranged on the trailer and a tractor coupling on the towing vehicle, said tractor coupling having locking elements for positive and rotatable grasping of substantially rotationally symmetrical surfaces of the kingpin, in a state in which lubricant-free and thus maintenance-free operation of the coupling device is possible.
Tractor couplings serve to connect trailer towing vehicles or tractor trucks in a positive but articulated manner to semi-trailers, together forming a so-called tractor-trailer unit.
Arranged on the towing vehicle is a coupling plate which is of substantially horseshoe-shaped configuration in plan view, so that a throat faces in the backward direction of the towing vehicle. A part, also horseshoe-shaped, called the wear ring is attached at the base of the throat. Located on either side of the wear ring are locking parts, in particular a so-called lokking hook, which initially (i.e. when the tractor coupling is open) are out of engagement with the throat opening.
Located on the trailer, on the underside of the front end, is a downward-pointing so-called kingpin, which in order to connect the towing vehicle and trailer is slid into the throat opening until it comes into contact with the wear ring. The locking parts are then brought into their closed position, and positively surround the kingpin. The surfaces of the wear ring, the locking hook, and the kingpin which thereby come into contact with one another respectively allow rotation of the coupling pin in the coupling plate. They are thus, to that extent, configured with rotationally symmetrical surfaces.
In the closed state, the wear ring and locking hook accordingly form a circular ring. The kingpin, received in the arrangement made up of locking hook and wear ring, has little or only minimal clearance in order to create a smooth connection between the trailer and truck.
The surfaces of the wear ring and locking hook which together wrap around the kingpin form contact surfaces for the kingpin.
During operation of the vehicle, in particular in curves, when the kingpin rotates in the arrangement made up of the wear ring and locking hook, correspondingly severe friction occurs between these parts, leading to frictional losses and wear on the kingpin, wear ring, and locking hook. After a corresponding period of operation, the diameter of the kingpin decreases as a result of wear.
Standards stipulate that a maximum reduction in kingpin diameter of approximately 2 mm is permissible, and the kingpin must then be replaced.
Tractor couplings of the kind set forth above are commonly known, and are widely used on a standard basis in tractor-trailer units.
An essential problem in tractor couplings of conventional type is friction between the mutually contacting surfaces of the towing vehicle on the one hand and the trailer on the other hand, especially in the contact region on the coupling plate and in the region of the kingpin. In conventional tractor couplings, therefore, a so-called plate lubrication system is used, in which wide lubricating grooves are provided for optimum greasing of the contact surfaces in the coupling plate. Because of the relatively large dimensions of coupling plates, the quantities of lubricating grease are correspondingly large.
It is known that conventional lubricated tractor couplings of this kind represent a considerable environmental problem, since large quantities of lubricating grease pass from these tractor couplings into the environment, and thus represent a hazard to, for example, groundwater.
Numerous tractor couplings are therefore already known in which so-called anti-friction coatings are used in the region of the coupling plate contact surface. Tractor couplings of this kind are referred to as "low-maintenance," since in them, only the bearing point of the kingpin in the locking member of the tractor coupling needs to be lubricated.
DE 35 30 467 A1 proposes, for a tractor coupling, to equip those surfaces of the wear ring and the locking hook that come into contact with the kingpin, with an anti-friction film. A polytetrafluoroethylene (PTFE) material was used as the anti-friction film. The anti-friction film consists more precisely of a PTFE-coated netting which is adhesively bonded onto the relevant surfaces of the wear ring and the locking hook, in a milled recess thereof.
Although PTFE possesses outstanding slip and anti-adhesion properties that are also familiar from other areas (coatings on cookware and clothes iron soleplates), one disadvantage of a PTFE anti-friction film for coupling plates is that the PTFE itself is subjected in practical use to extreme wear. In addition to perceptible creep behavior under the static load to which the PTFE film applied onto the wear ring and the locking hook is subjected (and which rapidly reduces tolerances to less than acceptable levels when PTFE is used on tractor couplings), PTFE offers practically no wear resistance with respect to abrasive materials.
But in a tractor coupling, because of the high surface pressure exerted by the kingpin on the wear ring and the locking hook, the anti-friction film must specifically be highly resistant to creep. In addition, if lubrication is to be partially or completely dispensed with (which is entirely desirable), the kingpin acts in highly abrasive fashion with respect to the PTFE, thus resulting in excessively rapid wear on the PTFE coating.
On the other hand, anti-friction films made of hard metal compounds such as titanium nitride have also been used for tractor couplings; these do protect the contact surfaces of the locking hook and wear ring from abrasion, but increase the abrasion of the kingpin to the same degree. The result is that the kingpin must also be equipped with an anti-friction film; but this considerably increases the expense.
DE-A-37 28 090 discloses a completely maintenance-free tractor coupling. In this known tractor coupling, the kingpin is held in a separable rolling bearing which is opened when the trailer is being coupled, and then closed again in the coupled position. The rolling bearing is lubricant-free, and allows the kingpin to rotate in the tractor coupling lock.
It is known from the aforecited DE-A1-43 04 857 to provide a thermal coating both in the region of the contact surface of the tractor coupling and in the region of the kingpin. This thermal coating consists of a thermally sprayed hard-material film, for example, a ceramic oxide, in particular aluminum oxide or titanium oxide, onto which a sliding agent film, in particular polytetrafluoroethylene, is applied. Coating systems of this kind are also known from other technology sectors, for example the technology of coating printing press cylinders, frying pans, and clothes iron soleplates.
Ceramic materials have the basic disadvantage that they are relatively brittle and thus sensitive to impact loads. Considerable impact loads can, however, occur in tractor couplings, especially when a trailer is being coupled to a towing vehicle.
Because of the considerable problems with regard to environmental protection, the idea has moreover arisen of making available methods with which conventionally lubricated tractor couplings can be brought into a state in which at least low-lubricant and thus low-maintenance operation is possible. In this connection, consideration has already been given to reworking tractor couplings that are already in operation by applying onto the contact surface of the plate the anti-friction coatings mentioned above, so as thereby at least to reduce the lubricant quantity required in a conventional surface lubrication system.
Modification into a completely maintenance-free state is not, however, possible therewith.